1. Field of the Invention
The present invention relates to high voltage electrical switchgear, and more particularly to high voltage load interrupters with safety systems.
2. Description of the Prior Art
Prior art high voltage load interrupters are generally of two types, viz., high voltage load interrupters which are oil immersed and which use oil as the interrupting medium (called herein "oil switches") and high voltage load or fault interrupters which are oil immersed and which use vacuum as the interrupting medium (called herein "vacuum switches").
The typical oil switch has the advantage that its contacts are visible through a window in the side of the oil-filled tank in which it is contained, and thus its operating state (open or closed) can be determined visually, insuring the safety of those who must work on the associated high voltage circuits.
However, as is well known in the prior art, arcing between the contacts of an oil switch carbonizes the oil in the tank, and after a small number of switch operations the carbonized oil becomes more and more opaque, ultimately reaching a state of opacity in which the contacts of the oil switch are no longer visible through the window in the tank wall.
Further, the carbonization of the oil in the tank produced by arcing between the contacts of the oil switch immersed therein creates explosive gasses and also reduces the insulation value of the oil.
Typically, the effective life of an oil switch is but a few hundred switch operations, i.e., interruptions, at most. A relatively few interruptions will carbonize the oil to such an extent that contacts are not visible.
As is also well known in the prior art, vacuum switches have many advantages over oil switches.
For example, the life of a vacuum switch is typically ten thousand operations. Further, since the arcing in a vacuum switch takes place inside a vacuum chamber, the surrounding insulating oil is not carbonized by this arcing.
However, vacuum switches have the disadvantage that their contacts are not visible through the window in the wall of the associated tank, due to the fact that the walls of the vacuum chambers containing the switch contacts are opaque.
A common safety practice of electric utility companies is to require that before a worker can work on a particular high voltage circuit that circuit must be isolated by a visible disconnect the contacts of which are directly viewable, and thus can be seen to be open.
As a result, the use of oil immersed vacuum switches has, in the past, required electrical utility companies to use hot stick removable terminations on high voltage cables in order to provide directly visible evidence of disconnection. However, the hot stick removal of such terminations is time-consuming, difficult, and in some cases results in failure of the insulating seals on the terminations when they are replaced on the switch bushings.
The term "prior art" as used herein or in any statement made by or on behalf of applicants means only that any document or thing referred to as prior art bears, directly or inferentially, a date which is earlier than the effective filing date hereof.
No representation or admission is made that a search has been made, or that no more pertinent information than that set out hereinabove exists.